SILVERTONE: 1472 Guitar Amplifier

The Silvertone 1472 is a small Sears/Danelectro valve combo using 2×12AX7, 2×6V6GT, a 6AU6 tremolo oscillator and a 6X4 rectifier, driving a 12” Jensen C12R speaker. 

SILVERTONE 1472 GUITAR AMP

The 1472 has separate microphone and instrument channels, each using one half of the first 12AX7. Both are fairly unusual gain stages, with large 330 kΩ plate resistors and 2.2 kΩ unbypassed cathode resistors. The two channels are then mixed through a pair of 330 kΩ resistors and fed into another 12AX7 gain stage, followed by a cathodyne phase inverter.

The output stage is a cathode-biased pair of 6V6GTs, running at around 260 V on the plates and 250 V on the screens, with a shared 220 Ω / 5 W cathode resistor. The schematic doesn't specify the output transformer impedance, although the operating conditions suggest something in the region of 8–10 kΩ plate-to-plate. There are also some very large 330 kΩ grid stoppers on the 6V6s, and no global negative feedback.

Tremolo is provided by a 6AU6 phase-shift oscillator. Rather than modulating one of the preamp stages, it varies the bias of both 6V6 output valves together.

The power supply is similarly old-school, with fairly small filter capacitors and a large 100 kΩ dropping resistor feeding the first 12AX7. That gives the sensitive input stages a fair bit of isolation from the rest of the amplifier, including the tremolo oscillator.

Overall, it's a simple but quite distinctive circuit: relatively low voltages, unusually high resistor values, a cathodyne phase inverter, bias-vary tremolo and no negative feedback. All of that should play a part in the amp's relatively early breakup and its particular character.

SILVERTONE: 1483 Guitar Amplifier

No, unfortunately, I did not buy one - just looking at the schematic, as it's a good candidate for conversion projects, outside of the normal Fender, Marshall, Vox realm.

It's about 23 watts, running a pair of 6L6 and it came with a single 15" speaker.   Two independent channels, with Volume, Bass and Treble on each.

The Silvertone 1483 is a fairly unusual mid-1960s valve amplifier.  It uses two independent 12AX7 input stages, each with its own passive bass and treble controls and volume pot, before the two channels are mixed through 330 kΩ resistors. A second 12AX7 then provides another gain stage followed by a cathodyne phase inverter.

Rather than driving the output valves directly from the phase inverter, there's an added a 6CG7/6FQ7 push-pull driver stage (which is a lot like a 12AU7), with 100 kΩ plate resistors and a shared 2.2 kΩ cathode resistor.  This then drives a cathode-biased pair of 6L6GCs, using a shared 270 Ω / 5 W cathode resistor (unbypassed).

The schematic gives around 350 V on the 6L6 plates, 345 V on the screens and 29 V at the cathodes, with B+ around 360 V.

The tone controls are also a little different from the usual Fender bass / treble arrangement.  Each channel uses a fairly elaborate passive network built around 220 kΩ resistors, 2.2 nF, 1.5 nF and 6 nF capacitors and 1 MΩ bass and treble pots.

The 1483 has an unusual “standby” arrangement.  Rather than switching off the B+ and keeping the heaters running, the standby switch shorts together the two opposite-phase signals feeding the 6L6 output valves.  The signals cancel, removing the differential drive to the push-pull output stage and effectively muting the amp while all valves remain powered normally.  

Overall, the 1483 is essentially two preamp channels feeding a common gain stage, cathodyne phase inverter, dedicated 6CG7 driver and push-pull 6L6 output stage — an unusual and quite sophisticated layout for a Sears/Silvertone amplifier of the period.

SILVERTONE 1483 - ORIGINAL SCHEMATIC

REDRAWN SCHEM - SINGLE CHANNEL

Note: I did not include the inductor across the speaker, as I have no plans to ever do anything like this (visible on image below from Mike and Mikes Guitar bar).